Hereinafter, a conventional receiver will be explained. Conventionally, when changing the modulation scheme in an on-going communication, the transmitter informs the receiver about the change in the modulation scheme and then changes the modulation scheme. However, in mobile communications there is a rougher change in the communication status so that the modulation scheme is changed without an advance notice to the receiver, and information transmission rate is flexibly changed, thereby increasing an average communication speed as well as enhancing confidentiality. Particularly in such a communication system as TDMA communications, where signals for reception are released in the form of bursts, the modulation scheme is judged from the data in the burst.
In the portable telephones that perform mobile communications, the transmitter performs an encoding process of combining the transmission data with specific codes for example. On the other hand, the receiver performs to the demodulated data a decoding process correspondingly to the encoding process performed, and corrects errors that have occurred during the transmission process. Through such an error correction, a certain level of communication quality is guaranteed.
It is well know that in the error correction process of demodulated data, a better quality of transmission can be achieved by soft decision decoding rather than by hard decision decoding. Here, the hard decision decoding yields a hard decision value, which is a result of determining what was in the transmission signal. The soft decision decoding yields a soft decision value, which is a combination of the hard decision value and reliability. For example, when the transmission side transmits a binary signal represented by 1 or −1, the hard decision value will be 1 or −1, whereas the soft decision value can be 0.9, −0.4 or the like. Here, the sign given to the soft decision value represents the hard decision value (i.e. the plus sign represents 1, and the minus sign represents −1), and the absolute value of the soft decision value represents the reliability, (Greater value indicates higher reliability).
According to the above mobile communications system, an interleaving process is performed to the encoded data on the transmission side. Therefore, even if errors are concentrated on a particular burst, these errors can be corrected as the errors can be dissipated in a deinterleaving process performed by the receiving device.
As described, in the mobile communications system such as PDC, IS-136, and GSM, the encoded data is transmitted in a plurality of bursts. Further, in the above system, the modulation scheme is changed for each coded block or every several blocks of the encoded data.
However, there is a problem in the conventional receiver in the mobile communications system. Specifically, when the modulation scheme is judged after receiving all of the bursts including the same coded blocks, all of the received signals in the bursts must be stored, which requires a large circuit size.
On the other hand, there is another problem when the modulation scheme is judged burst by burst and the judgment value obtained after the demodulation is stored. Specifically, the circuit size (memory capacity) can be smaller than in the above instance of storing all the received signals of the bursts, but due to an increased risk of performing a demodulation process in an incorrect mode, the received coded blocks sometimes cannot be decoded correctly due to this error.
It is an object of the present invention to provide a receiver capable of improving error correction ability while reducing the circuit size.